Method for distilling a ternary azeotrope of fluorosilicic acid, hydrogen fluoride and water

ABSTRACT

Fluorosilicic acid solutions, which normally undergo decomposition when distilled, thereby creating unwanted forms of SiO 2 , are rendered stable during distillation by providing in the fluorosilicic acid solution an amount of HF which is at least about 10 parts of HF per 36 parts of H 2  SiF 6  and an amount of H 2  O which is at least about 54 parts of H 2  O per 36 parts of H 2  SiF 6 . The mixture is distilled to remove any excess H 2  O and excess HF that is present, without encountering formation of SiO 2 , until an azeotropic solution containing about 36% H 2  SiF 6 , about 10% HF and about 54% H 2  O is reached. The ternary azeotrope, being of constant quality and concentration, is more suitable for use in various processes, such as processes for making fumed SiO 2 , than H 2  SiF 6  solutions which are not of constant quality or concentration.

BACKGROUND OF THE INVENTION

There exist aqueous solutions of H₂ SiF₆, such as waste streams fromprocessing of ores or minerals that contain silicon and fluorine values,which would be useful in chemical processes, such as in preparing fumedSiO₂ and HF, if the H₂ SiF₆ could be purified to a constant quality bydistillation. Attempts to distill water or other low-boiling materialfrom relatively weak solutions of H₂ SiF₆, in order to concentrate orpurify the H₂ SiF₆, cause decomposition and create unwanted forms ofSiO₂. Attempts to distill H₂ SiF₆ away from higher boiling impurities,such as H₃ PO₄ or H₂ SO₄, also cause decomposition and create unwantedforms of SiO₂. The SiO₂ so formed has no significant commercial value,thus the commercial significance of H₂ SiF₆ is substantially negated bythe formation of relatively useless forms of grainy-type SiO₂. If theaqueous H₂ SiF₆ can be converted to a relatively constant quality andconcentration, it can be used in processes which form commerciallysignificant products, such as being combusted to make fumed SiO₂ and HF.Fumed SiO₂ is a form of SiO₂ which has good commercial value.

There exists then, a need for converting aqueous solutions of H₂ SiF₆,especially those solutions which are not of constant quality orconcentration, to solutions which can be distilled to relativelyconstant quality and concentration without encountering thedecomposition of H₂ SiF₆ and without creating unwanted forms of SiO₂.

As further background to certain embodiments of the present invention,there are a number of references which teach the burning of siliconfluoride compounds, such as SiF₄ and H₂ SiF₆, by injecting the siliconfluoride compounds into a combusted mixture of fuel and oxygen in orderto prepare fumed SiO₂. The oxygen is generally supplied as air and,conveniently, the fuel may be hydrogen. The fuel also may be an easilycombusted hydrocarbon such as an aliphatic hydrocarbon having from 1 toabout 12 carbon atoms or mixtures of such hydrocarbons. Representativepatents for such formation of fumed SiO₂ are, for example, U.S. Pat.Nos. 2,819,151; 3,203,759; 3,110,562 and others. It is highly desirablethat the feed stream of H₂ SiF₆ be of substantially constant quality andconcentration in order that the process not be erratic, especially sincesuch processes are normally expected to be performed continuously.

Throughout this disclosure, the "decomposition" which is avoided by thepresent invention is illustrated by:

    3H.sub.2 SiF.sub.6 (aq.) .sup.vaporization 3SiF.sub.4 (↑)  + 6HF(↓),

then in the distillation condenser:

    3SiF.sub.4 + 2H.sub.2 O(1) → SiO.sub.2 (grainy)+2H.sub.2 SiF.sub.6.

it is an object of the present invention to provide an aqueous solutionof H₂ SiF₆ which is of substantially constant quality and concentrationfor use as feed material to chemical processes.

It is another object to provide an aqueous solution of H₂ SiF₆ which canbe distilled without encountering decomposition and without creatingunwanted forms of SiO₂.

It is still another object to provide an aqueous solution containing H₂SiF₆ of substantially constant quality and concentration for use in ahigh temperature combustion process in which fumed SiO₂ and HF areproduced.

Another object is the removal of substantial amounts of water from weaksolutions of H₂ SiF₆ by distillation at elevated temperature to make amore concentrated solution without encountering decomposition andcreation of grainy-type SiO₂.

These and other objects are obtained by the invention described andclaimed herein.

SUMMARY OF THE INVENTION

It has now been found that the formation of unwanted SiO₂ which normallyforms in the condenser when aqueous solutions containing H₂ SiF₆ aredistilled can be avoided by providing, for each 36 parts of H₂ SiF₆, atleast about 10 parts of HF and at least about 54 parts of H₂ O in thesolution. Such a solution can be distilled, the low boiling componentsboiling off first (including excess water and/or HF or other "lights"such as HCl) and then the ternary azeotrope containing about 36% H₂SiF₆, about 10% HF, and about 54% water can be distilled away from thehigher boiling impurities, such as H₃ PO₄ or H₂ SO₄ or other highboiling or non-volatile impurities.

The ternary azeotrope may then be used as feed for chemical processeswhich utilize H₂ SiF₆, such as processes which burn the H₂ SiF₆, by thecombustion of fuel and oxygen to make fumed SiO₂ and HF. Fumed SiO₂ is acommercially desirable form of SiO₂.

Alternatively, the distillation process to obtain the desired azeotropeis operated in two distillation steps, either batchwise or continuously,by performing the first step in one distillation operation at reflux inwhich the volatiles and any excess H₂ O or HF are boiled off whiletransferring the heavies to a second distillation to boil the azeotropeaway from any non-volatiles or higher boiling impurities that may bepresent.

Throughout this disclosure, all percents, ratios, and quantities are byweight.

DETAILED DESCRIPTION OF THE INVENTION

An aqueous solution of H₂ SiF₆ is provided with sufficient HF and H₂ Oto give a solution containing at least about 10 parts by weight of HFand at least about 54 parts by weight of H₂ O per 36 parts of H₂ SiF₆.The aqueous mixture is then distilled to remove the low boilingcomponents, including any excess HF and/or H₂ 0 that may be present. Bythe term "excess", it is meant that more water and/or more HF may be inthe solution, prior to distillation, than is required to form theternary azeotrope of about 36% H₂ SiF₆, about 10% HF, and about 54% H₂O.

The ternary azeotrope boils at about 116° C, ± 2°, at ambientatmospheric pressure. The composition of the azeotrope is difficult toanalyze exactly, due to lack of a really good analytical technique,therefore the concentrations are given as "about" and are representativeof nominal amounts as determined from many samples. The analyticaltechnique for measuring the compositions of the azeotrope is done by amethod of cold and hot titrations with standard NaOH, using prescribedindicators, in accordance with the procedures described by W. W. Scottin "Standard Methods of Chemical Analysis," Fifth edition, Vol. 11, p.2209, New York, D. Van Nostrand Co. (1939), and by W. B. Sherry, et al.Ind. Eng. Chem., Anal. Ed. 16, p. 483 (1944).

The aqueous solution of H₂ SiF₆ with which one starts can be anysolution containing significant quantities of H₂ SiF₆. Solutions whichcontain more water than is needed to form the azeotrope are usableprovided that enough HF is supplied. There is no problem with havingmore HF and/or water than is needed to form the azeotrope. The problemwith not having enough HF is that the deficiency results,correspondingly, in the formation of unwanted forms of SiO₂ when thesolution is distilled.

If the solution has enough (or more than enough) HF, but not enoughwater, some of the HF and H₂ SiF₆ will boil off during distillation.Eventually, in this case, the concentration of H₂ SiF₆ will decrease tothe point at which the ratio of water to H₂ SiF₆, and of HF to H₂ SiF₆,is the proper ratio for the ternary azeotrope, but the H₂ SiF₆ goingoverhead in the meantime is subject to decomposition, in this caseyielding HF + SiF₄.

The following example demonstrates the present invention in comparisonwith an attempt to distill an aqueous solution of H₂ SiF₆ which does nothave the required minimum amount of HF to form the azeotrope.

EXAMPLE 1

An aqueous waste stream from a mineral processing plant, containingabout 25% H₂ SiF₆ and a small percent of heavies (mostly H₂ SO₄ and H₂PO₄), is fed to a distillation column in an attempt to distill off waterand form a more concentrated aqueous solution of H₂ SiF₆. It is soonfound that a grainy form of SiO₂ is being formed and is plugging theoverhead condenser and lines and aqueous H₂ SiF₆ is coming over. This isevidence that some of the H₂ SiF₆ is undergoing decomposition to SiF₄and HF, and the SiF₄ is reacting with water in the overhead to giveSiO₂.

The same waste stream, with about 10% HF added to it, is fed to thedistillation column. Excess water and HF distill over leaving anazeotrope containing about 36% H₂ SiF₆, about 10% HF and about 54% waterwhich boils at about 116° C. No decomposition of the H₂ SiF₆ is foundand no SiO₂ is formed. The ternary azeotrope is then distilled from thehigher boiling heavies and during the distillation of the azeotrope, nodecomposition is found and formation of unwanted SiO₂ is avoided.

The azeotrope is fed into the flame in a combustion chamber where amixture of hydrogen and air is burned. The hot gases from the combustionchamber are immediately passed through a solids separator where thefumed SiO₂ product is collected and removed. The HF and any otherfluoride values present in the gases which have passed through theseparator are cycled back to the aqueous waste stream for further use inan azeotropic mixture.

EXAMPLE 2

An aqueous solution of H₂ SiF₆ with HF added to it is distilled asfollows:

238 Parts of solution containing about 8.3% HF, about 26.3% H₂ SiF₆, andabout 65.4% H₂ O are distilled in a distillation column equipped with aninner condenser to provide reflux.

After about 54 parts of overhead has been collected a sample iscollected for analysis (Sample A).

After about 100 parts of overhead has been collected and as the pottemperature begins to line-out at about 116° C, ±2° the distillate iscut in order to begin collecting the ternary azeotrope and a sample ofthe first cut is taken for analysis (Sample B).

The next 35 parts of overhead, coming over at about 116° C, ±2°, issampled for analysis (Sample C).

The remaining overhead fractions (coming over at about 116° C, ±2°) arecombined with the 35 parts and a sample of the composite is sampled foranalysis (Sample D). No SiO₂ is formed during distillation.

    ______________________________________                                        Analysis                                                                      ______________________________________                                        Sample                                                                              %HF     %H.sub.2 SiF.sub.6                                                                     %H.sub.2 O  Remarks                                    ______________________________________                                        A     0.6     0        99.4   excess H.sub.2 O and HF coming                                                over                                            B     4.13    0.51     95.36  as temp. reaches b.p. of                                                      azeotrope, some H.sub.2 SiF.sub.6                                             comes over                                      C     10      35       55     azeotropic mixture                              D     10      35.2     54.8   azeotropic mixture                              ______________________________________                                    

This example demonstrates that the excess H₂ O and HF boils off beforethe azeotropic mixture distills over, yet no SiO₂ forms in the overheadindicating decomposition of the H₂ SiF₆.

We claim:
 1. A method of distilling an aqueous solution containing H₂SiF₆ while avoiding the formation of unwanted SiO₂ during thedistillation, said method comprisingproviding in said solution at leastabout 10 parts by weight of HF and at least about 54 parts by weight ofH₂ O for each 36 parts by weight of H₂ SiF₆, subjecting the solution todistillation to remove low boiling components, and continuing saiddistillation to separate the ternary azeotrope comprising about 36% H₂SiF₆, about 10% HF, and about 54% H₂ O from any higher boiling ornon-volatile components.
 2. The method of claim 1 wherein the aqueoussolution of H₂ SiF₆ is a waste stream from a mineral treating process.3. The method of claim 1 wherein the aqueous solution of H₂ SiF₆contains, as impurity, at least one mineral acid selected from the groupconsisting of HCl, H₂ SO₄ and H₃ PO₄.
 4. The continuous distillation ofan azeotrope containing about 36% H₂ SiF₆, about 10% HF and about 54% H₂O by the method which comprisescontinuously supplying an aqueous streamof H₂ SiF₆ to a first distillation operating at a reflux with the pottemperature at about 114°-118° C, said stream of H₂ SiF₆ containing atleast about 10 parts by weight of HF and at least about 54 parts byweight of H₂ O per 36 parts by weight of H₂ SiF₆, taking off theoverhead vapors comprising whatever volatiles and excess H₂ O and HF arepresent which are not required by the azeotrope, while continuouslytransfering the heavies from the first distillation to a seconddistillation, said heavies comprising about 10 parts by weight of HF andabout 54 parts by weight of H₂ O per 36 parts by weight of H₂ SiF₆ alongwith any higher-boiling or non-volatile ingredients contained therein,distilling from the second distillation the desired azeotrope having aboiling temperature of about 114°-118° C and drawing off thehigher-boiling or non-volatile ingredients from the lower portion of thedistillant.
 5. The continuous distillation of claim 4 wherein theaqueous stream contains minor amounts of at least one mineral acidselected from the group consisting of HCl, H₂ SO₄ and H₃ PO₄.